Kaolin is widely employed as a pigment in the manufacture of paper to improve the printing characteristics of the paper and to provide a glossy, white, opaque finish. It has long been recognized that the addition of TiO.sub.2 (titanium dioxide) to paper coating formulations comprised of kaolin, a dispersing agent, adhesives, and other additives improves the opacity of the coating film. Opacity in paper coatings is caused by the light scattering ability of the pigment which can be estimated by the scattering coefficient as described in Tappi, 1978, Vol. 61, No. 6, p. 78-80.
Several modified kaolin products have been developed to replace, at least partially, the more expensive TiO.sub.2. These modification treatments generally fall into three categories. Firstly, kaolin may be calcined at a temperature above 900.degree. C. to produce an opacifying material such as described in U.S. Pat. Nos. 4,078,941 and 4,678,517. A second method, such as described in U.S. Pat. Nos. 4,075,030, 4,738,726, and 4,076,548, involves treatment of kaolin with organic reagents which chemically bridge between particles and flocculate the fine particles. A third method of producing an opacifying pigment, described by International Patent Application No. PCT/US86/00158, involves treating kaolin with a metal chloride.
In general, the just described methods seek to structure the kaolinite particles into agglomerates which incorporate light scattering voids in the pigment. These structured pigments must preserve their light scattering voids during paper coating and calendering, a requirement which limits the use of weakly flocculated kaolins. Pigments with increased light scattering characteristics, compared with untreated kaolin, have sometimes been termed "high bulking" owing to this presence of voids in the kaolin particle structure.
The goal in all of these product developments has remained the same, namely to provide inexpensive pigments, compared to titanium dioxide, that may be relatively sparsely employed for even further economy, and yet which nevertheless provide good opacity, brightness and reflectance as coating and filler materials, particularly for use on paper.